In various nondestructive testing practices, an object under evaluation has a time-varying, i.e., non-zero frequency, signal applied thereto as furnished by a system oscillator. A signal is derived from the object, which signal has a carrier with modulation in accordance with object characteristics. Detection circuitry typically has a filter tuned to the oscillator frequency, an amplifier to increase signal level to a useful level, and circuitry receiving the filtered and amplified signal and developing two signals therefrom indicating its constituency at different phases. In general, the zero degree phase (sine) and the ninety degree phase (cosine) are selected and the two developed signals are then called the in-phase and quadrature components of the detected object signal and provide a complete set of information in the one frequency at hand.
Departures from true ninety-degree separation between the in-phase and quadrature signals, called "quadrature error", results in output information which is inaccurate, since microprocessors and the like, processing the in-phase and quadrature signals to develop output information, necessarily treat same as having true ninety-degree separation.
In the detection circuitry, phase shift is introduced by required filtering and amplification of the signal derived from the object, giving rise to quadrature error both from resulting phase difference in in-phase itself as against oscillator zero-degree phase and from other than true ninety-degree separation between in-phase and quadrature.
A typical frequency-based detection system of the type described above is the basic eddy current system used to determine characteristics, such as electrical conductivity, of metal stock. Here, a coil defines a central passage for insertion of a known sample of the stock and is excited by an oscillator. A detector receives an input signal from the coil and yields in-phase and quadrature output signals. Polar presentation information is developed from these output signals, i.e., a vector magnitude and an angle, and an oscilloscope presents the corresponding polar display.
A so-called "balancing" practice is required at the outset of use of such eddy current system and the general system as well. In this practice, one forces the system into a subrange of the overall operational range of the system, such that signal processing to the point of generation of the in-phase and quadrature output signals occurs without confronting saturated circuitry. The subrange is thus to extend from the system origin in X and Y. Typically, the actual system null, prior to balancing, is substantially off-null in X or Y or both.
In present practice, balancing is done manually by an operator. The operator moves the standard object through the coil and adjusts a potentiometer while observing the display screen until a potentiometer setting is reached at which the standard object provides a display which stays in-range throughout object movement in the coil. This practice, while time-consuming and burdensome, effects an injection into the detector circuitry of a fixed offset voltage for summation with the object derived signal to avert quadrature error attending system operation in saturated range.
Even given this laborious initial system setup, quadrature error arises from X and Y gain differences inherent in the operating subrange. In applicant's view, the art has not heretofore provided method and system achieving quadrature correction in degree providing comfort to the user demanding highly accurate output information from frequency-based object examination systems. Nor, in applicant's view, has this field of endeavor heretofore realized meaningful benefit of microprocessor capability currently available in addressing quadrature error elimination. Finally, in respect of such microprocessor capability, applicant is of the persuasion that artisans in the subject field are remiss in not realizing computer-assist to the extent of validating system generated information.